1. Field of the Invention
This invention relates to a length measuring system comprising a scale member provided with a scale and a scanning unit, which is movable along the scale and is guided by the scale member or by separate tracks, which are parallel to the scale and is coupled to an actuating member by a backlashless coupling, which has coupling elements, which are respectively connected to the scanning unit and to the actuating member and permit a movement of the actuating member relative to the scanning unit in a direction which is transverse to the scale and which are substantially in point contact with each other on engaging surfaces which are substantially curved about crossing axes which are normal to the tracks, and biasing elements are interposed between the actuating member and the scanning unit and urge the coupling elements against each other.
2. Description of the Prior Art
The length measuring system of the kind to which the invention relates may be so-called absolute-value measuring systems having a coded scale or incremental measuring systems in which the scale provided on the scale member consists of a bar pattern or other combination of bright and dark fields. The scale may be scanned by the scanning unit by optical, optoelectronic, capacitive, inductive or magnetic methods. In each case, the scale member and the scale will be designed to be suitable for the scanning method which is employed. The actuating member serves to connect the measuring system to a member which has a length or performs a movement which is to be measured. Such part may consist, e.g., of a tool carriage, which moves in a machine tool along a predetermined axis. As it is virtually impossible to ensure that the track for guiding such carriage is absolutely straight and/or absolutely parallel to the scale of the measuring system and vice versa, the actuating member must be able to move also transversely to the scale in such a manner that the actuating member and the scanning unit will not be caused to move relative to each other along the scale. That requirement can be met by the provision of coupling arrangement of the kind described hereinbefore.
AT-B-382,966 discloses a length measuring system of the kind described first hereinbefore. The object underlying that known length measuring system resides mainly to permit corrections to be made during the measuring operation in that the scanning unit is adjusted relative to the actuating member as the scanning unit is moved along a template. This is accomplished in that one coupling element consists of a vertical pin and the other coupling element consists of an eccentrically mounted roller, which extends transversely to the pin, and said coupling elements are urged against each other by a spring. Another spring biases an arm, which carries a roll carrier and which scans a template so that the eccentric roller is rotated in dipendence on the correcting template and corrections are thus introduced into the measuring operation.
DE-A-32 01 887 discloses a backlashfree coupling, which differs in kind from the couplings of the kind described hereinbefore and in an articulated joint having two degrees of freedom for translational movements in directions which are at right angles to the scale are arranged one behind the other. The linear guiding means may consist of a magnetic plate and of a member which has a spherical surface in contact with said plate.
It is known from DE-A-2 349 944 that a scanning unit can directly be guided on a scale member of glass and may be coupled to an actuating member by a leaf spring, which has a ball end extending into a socket formed in the scanning unit and exerts a resulting force acting along a space diagonal of the scale member to urge the ball into the socket and to urge the track-engaging means of the scanning unit against the scale member. In other embodimens of that basic design an additional guidance of the scanning unit is effected by means of rollers, which may alternatively engage track surfaces which are parallel to the scale and provided in an enclosure that contains the scale member.
From the periodical "Messen and Prufen/Automatik", July/August 1974, pages 435 to 439, it is known to provide a coupling consisting of two crossed rollers, which are urged against each other by springs. In other couplings the connection is established by non-buckling wires and leaf springs.
In the designs desclosed in DE-A-25 10 219 and DE-B-28 45 542, one coupling element carries a hardened metal ball and the other coupling element comprises a hard metal member, which has a planar surface that is normal to the scale, and springs are provided by which the two coupling elements are urged against each other. Said springs may consist of multiple-curvature spring clips, which engage the two coupling elements. In addition to said spring clips, coil springs which act on the associated tracks approximately normal to the line of action of the track-engaging elements of the scanning unit and are optionally adjustable and serve to urge the track-engaging elements against the associated tracks.
The known couplings have high manufacturing costs and are bulky and require great care to be taken in the mounting of the springs by which the coupling is held against backlash and of the springs which urge the scanning unit against the tracks. A basic disadvantage of arrangements which comprise springs resides in that all springs which may be employed may be caused to vibrate and for this reason the pressure forces which must be exerted to urge the coupling elements against each other and to urge the track engaging elements against the tracks are often much stronger than is required to ensure a coupling of the actuating member to the scanning unit and a satisfactory guidance of the scanning unit on the associated track surfaces as the scanning unit is usually very light in weight.